1. Field of the Invention
The present invention relates to a wireless communication system for making space-division multiple access, a weight control apparatus, and a weight vector generation method.
2. Description of the Related Art
In an FWA (Fixed Wireless Access) system, a base station and fixed terminal station make wireless, high-speed communications. In a point-to-multipoint (P-MP) system, a base station communicates with a plurality of terminal stations. In the P-MP system, space-division multiplex access (SDMA) is known as means for increasing the subscriber capacity. As shown in FIG. 16, an adaptive array 1002 arranged in a base station 1001 forms orthogonal beams that do not interfere with each other. These beams allow simultaneous communications of a plurality of terminals 1003.
FIG. 17 shows an arrangement of the adaptive array 1002 when the multiplexing degree=2. In beam forming circuits 1051 and 2051, appropriate weight vectors are set in weighting devices 1501 to 1504 and 2501 to 2504. These weighting devices and combiners 1512 and 2512 weight and combine the outputs from antenna elements 1011 to 1014, thereby forming orthogonal beams having maximum directionality toward one terminal, and null directionality toward the other terminal.
The same applies to a case wherein the multi-plexing degree is more than 3. That is, orthogonal beams are formed to have maximum directionality toward the objective terminal, and null directionality toward a plurality of remaining terminals.
In the conventional system, weights for forming orthogonal beams used to attain space-division multiplex access are individually calculated and held in correspondence with all combinations of terminals while considering the multiplexing degree for a given number of base stations. For example, when the multiplexing degree=2 and the total number of terminals=n, the number of combinations of weights to be calculated and held as combinations of terminals is n×(n−1). Therefore, the number of combinations of weights to be held becomes huge with increasing number of registered terminals which must undergo space-division multiplex access.
A wireless communication system that makes packet communications by CSMA/CA is known.
FIG. 22 shows the arrangement of an IEEE802.11 wireless LAN system using CSMA/CA. A base station 900 senses carriers before transmission of packets to a terminal 911. Upon receiving packets containing channel reserve information from a terminal 913, the base station 900 postpones packet transmission during that reserved period. After that, the base station 900 waits for a random time period (back-off process) calculated by a controller 901, and transmits packets addressed to a target terminal 911. If the received data is correct, the target terminal 911 transmits an acknowledge response packet (ACK) to the base station after an elapse of a predetermined period of time. If the base station 900 fails to receive ACK from the target terminal 911 after an elapse of a predetermined period of time, it executes the back-off process using the controller 901 and transmits data again.
On the other hand, an adaptive array can improve communication quality by forming beams that reduce interference from a neighboring cell, as shown in FIG. 23.
The adaptive array generally makes beam control on the basis of a received signal. For example, a direction-constrained power minimization method suppresses all received signals as interfering signals while holding the gain in a specific direction. Therefore, if a situation in which only an interference wave from a neighboring cell arrives is formed, and the direction-constrained power minimization method is used in this situation, beams that can cover a specific area in the self cell and can remove the interference wave can be formed.
However, in order to suppress all co-channel interference waves from a neighboring cell, the number of antenna elements must be increased, resulting in a large apparatus scale.
Since the IEEE802.11 wireless LAN system using CSMA/CA does not make integrated control of packet transmission, as described above, it is difficult to form a situation in which only terminals or base stations to be suppressed transmit packets. As a result, a terminal of the self cell transmits packets in place of the terminal or base station to be suppressed, and unwanted beams that suppress such packets are formed.